XAFS study of solid-solid transitions under high pressure

XAFS is a very powerful short-range structural probe, which complements the popular long-range structure probing x-ray diffraction technique. It is utilized to study pressure induced phase transitions of iron, tin iodide (SnI{dollar}sb4{dollar}) and germanium iodide (GeI{dollar}sb4{dollar}) in this thesis.; Iron undergoes a bcc-hcp transition at 13 GPa, which is believed to be a martensitic transition. XAFS study of this transition is to complement previous x-ray diffraction studies and to investigate the distorted hcp structure during the transition reported by Bassett and Huang. XAFS spectra of the iron k-edge are measured under pressure up to 20.0 GPa at SSRL. Our analysis yields similar lattice constants of the hcp phase as the Bassett and Huang's study, and an anomalously large lattice constant for the bcc phase for the first time in the final stage of the forward transition (17 GPa), and persisting until 12 GPa upon release of the pressure. These anomalies are attributed to interfacial strains between the bcc and hcp phases, and a possible intermediate fcc structure during the transition based on discussions of three possible transition models.; SnI{dollar}sb4{dollar} undergoes a phase transition between 9 and 18 GPa. The transformation is simultaneously from crystalline to amorphous and from insulator to conductor. GeI{dollar}sb4{dollar} has a similar transition at a slightly higher pressure between 15 and 22 GPa. XAFS spectra of Sn k-edge and I k-edge in SnI{dollar}sb4{dollar} and GeI{dollar}sb4{dollar} are measured under pressure up to 20.0 GPa.; We observe a mixture of crystal and amorphous SnI{dollar}sb4{dollar} and GeI{dollar}sb4{dollar} phases from the onset of the transition up to the highest pressure in our experiment. We find that the amorphous phase has weaker intramolecular Sn-I and Ge-I bonds and stronger intermolecular I-I bonds, and has much larger Sn-I and Ge-I bond length disorders than the crystal phase. This suggests that the SnI{dollar}sb4{dollar} and GeI{dollar}sb4{dollar} molecules are weakened and distorted in the amorphous phase, and that the I-I intermolecular interaction assumes the dominant role determining the properties of the sample. The networking of I-I bonds changes SnI{dollar}sb4{dollar} and GeI{dollar}sb4{dollar} from insulator to conductor as suggested by Pasternak et al.